Coriolis-type mass flowmeter

ABSTRACT

A mass flowmeter of the Coriolis-type in which the fluid to be metered is conducted through a loop supported on a stationary frame, the loop having a natural resonance frequency. Mounted at the vertex of the loop is a ballistic vibrator which is electrically energized to cause the loop to vibrate at its natural frequency on either side of its static plane. The fluid passing through the loop is subjected to Coriolis forces, causing the vibrating loop to torsionally oscillate in accordance with the mass flow rate of the fluid. This torsional oscillation is sensed by a pair of strain gauge transducers mounted in balanced relation on opposite legs of the loop, whereby the signals yielded by the transducers have a difference in magnitude therebetween that depends on the amplitude of the torsional oscillation. These signals are applied to a differential amplifier whose output is proportional to the mass flow rate of the fluid.

RELATED APPLICATION

This application is a continuation-in-part of copending application Ser.No. 831,564, filed Feb. 21, 1986, now U.S. Pat. No. 4,691,578 of thesame title, whose entire disclosure is incorporated herein by reference.

BACKGROUND OF INVENTION

1. Field of Invention

This invention relates generally to mass flowmeters, and moreparticularly to a Coriolis-type meter of simplified construction.

2. Status of Art

A mass flow rate meter is an instrument for measuring the mass of afluid flowing through a conduit per unit time. Most meters for thispurpose measure a quantity from which the mass can be inferred, ratherthan measuring mass directly. Thus, one can measure the mass flow ratewith a volumetric flowmeter by also taking into account pressure,temperature and other parameters to compute the mass.

A Coriolis-type mass flowmeter, which is also called aCoriolis/Gyroscopic meter, provides an output directly proportional tomass flow, thereby obviating the need to measure pressure, temperature,density and other parameters. In this type of meter, there are noobstacles in the path of the flowing fluid, and the accuracy of theinstrument is unaffected by erosion, corrosion or scale build-up in theflow sensor.

The theory underlying a Coriolis-type mass flowmeter and the advantagesgained thereby are spelled out in the article by K.O. Plache,"Coriolis/Gyroscopic Flow Meter" in the March 1979 issue of MechanicalEngineering, pages 36 to 39.

A Coriolis force is generally associated with a continuously rotatingsystem. Thus, the earth's rotation causes winds from a high pressureregion to spiral outwardly in a clockwise direction in the northernhemisphere, and in the counterclockwise direction in the southernhemisphere. And a person moving on a merry-go-round will experience alateral force and must lean sideways in order to move forward whenwalking outward along a radius.

A Coriolis force and precession in a gyroscope arise from the sameprinciple. In a gyroscope, when a torque is applied at right angles tothe axis of rotor spin, this will produce a precessional rotation atright angles to the spin axis and to the applied torque axis. A Coriolisforce involves the radial movement of mass from one point on a rotatingbody to a second point, as a result of which the peripheral velocity ofthe mass is caused to accelerate. This acceleration of the massgenerates a force in the plane of rotation which is normal to theinstantaneous radial movement.

In one known form of Coriolis-type mass flowmeter, the fluid to bemetered flows through a C-shaped pipe which, in association with a leafspring, act as the opposing tines of a tuning fork. This fork iselectromagnetically actuated, thereby subjecting each moving particlewithin the pipe to a Coriolis-type acceleration. The resultant forcesangularly deflect or twist the C-shaped pipe to a degree inverselyproportional to the stiffness of the pipe and directly proportional tothe mass flow rate within the pipe.

The twist of the pipe is electro-optically sensed twice during eachcycle of tuning fork oscillation which takes place at the naturalresonance frequency of the structure. The output of the optical detectoris a pulse whose width is modulated as a function of the mass flow rate.This pulse width is digitized and displayed to provide a numericalindication of mass flow rate.

In the Roth U.S. Pat. No. 3,132,512, a Coriolis-type mass flowmeter isdisclosed in which a flow loop vibrating at its resonance frequency iscaused to oscillate about a torque axis which varies with fluid flow inthe loop. This torsional oscillation is sensed by moving coiltransducers.

The Cox et al. U.S. Pat. No. 4,192,184 shows a Coriolis-type meterhaving two U-shaped flow loops arranged to vibrate like the tines of atuning fork, torsional oscillation of these loops in accordance with themass of the fluid passing therethrough being sensed by light detectors.In the Smith U.S. Pat. No. 4,222,338, electromagnetic sensors provide alinear analog signal representing the oscillatory motion of a U-shapedpipe. Electromagnetic sensors are also used in Smith et al. U.S. Pat.No. 4,491,025 in which the fluid whose mass is to be measured flowsserially through two parallel U-shaped pipes which together operate asthe tines of a tuning fork.

In prior art mass flowmeters of the above-described Coriolis type, theflow loops are caused to vibrate at their natural resonance frequency byan electromagnetic transducer, one element of which is mounted on theflow loop so that it is movable relative to the other element which issupported on a stationary frame or platform, the loop element beingattracted or repelled by the platform element. Thus, in the above-notedSmith patents, the transducer is constituted by a coil mounted on theflow loop and a cooperating permanent magnet mounted on a fixedplatform. This type of arrangement is troublesome, for unless the frameor platform is highly stable, the flow loop will not be driven at aconstant amplitude, and the output of the meter will not be accurate.

The same problem exists in regard to the transducer which senses thetorsional oscillation of the flow loop, for again, one element of thetransducer is secured to the loop, and the other to a fixed frame.

The need for a stable frame or platform as a frame of reference for amoving element in a transducer as in prior art Coriolis-type massflowmeters makes such meters mechanically more complex and moreexpensive to manufacture. Moreover, prior art systems are relativelycumbersome.

SUMMARY OF INVENTION

In view of the foregoing, the main object of this invention is toprovide a Coriolis -type mass flowmeter of compact, simplifiedconstruction which operates efficiently, reliably and accurately.

More particularly, an object of this invention is to provide a massflowmeter of the above-type which makes use of a single turn flow loopthat is excited into vibration at its natural resonance frequency by avibrating ballistic mass which is independent of a reference frame orplatform.

Also, an object of this invention is to provide a mass flowmeter inwhich the torsional oscillation of the vibratory flow loop is sensedwithout reference to a stable frame or platform.

A significant advantage of the invention is that acceleration effectsonly act on the flow loop and not, as in prior art systems, also on theframe or platform serving as a reference for the transducers included inthe system. In prior art systems, acceleration effects will remainunless the whole mechanical system is balanced, whereas no suchrequirement exists in a system in accordance with the invention.

Briefly stated, these objects are attained in a mass flowmeter of theCoriolis type in which the fluid to be metered is conducted through aloop supported on a stationary frame, the loop having a naturalresonance frequency. Mounted at the vertex of the loop is a ballisticvibrator which is electrically energized to cause the loop to vibrate atits natural frequency on either side of its static plane. The fluidpassing through the loop is subjected to Coriolis forces, causing thevibrating loop to torsionally oscillate in accordance with the mass flowrate of the fluid. This torsional oscillation is sensed by a pair ofstrain gauge transducers mounted in balanced relation on opposite legsof the loop, whereby the signals yielded by the transducers have adifference in magnitude therebetween that depends on the amplitude ofthe torsional oscillation. These signals are applied to a differentialamplifier whose output is proportional to the mass flow rate of theliquid.

OUTLINE OF DRAWINGS

For a better understanding of the invention as well as other objects andfurther features thereof, reference is made to the following detaileddescription to be read in conjunction with the accompanying drawings,wherein:

FIG. 1 is a block diagram of a Coriolis-type mass flowmeter inaccordance with the invention;

FIG. 2 is an end view of the flow loop assembly;

FIG. 3 is a schematic illustration of the ballistic vibrator included inthe assembly;

FIG. 4 illustrates the flow loop in side view;

FIG. 5 illustrates how the flow loop vibrates with and without flow; and

FIG. 6 shows the angle made by the loop during its harmonic vibration.

DESCRIPTION OF INVENTION Structure

Referring now to FIGS. 1 and 2, there is shown a Coriolis-type massflowmeter in accordance with the invention which includes a single-turnflow loop 10 formed by a pipe having a pair of arcuate legs 10A and 10B.The loop is supported on a rigid stationary frame 11 provided with amidsection 12 and left and right end sections 13 and 14.

Leg 10A of the loop leads into a horizontal pipe section 10C whichpasses through a bore in end section 13 of the frame and terminates in acoupling flange 15. Leg 10B of the loop leads into a horizontal pipesection 10D parallel to pipe section 10C, pipe section 10D passingthrough a bore in end section 14 of the frame and terminating in acoupling flange 16. As indicated by the term "connection" in FIG. 1, themidpoint of loop 10 is secured to the midsection 12 of frame 11, pipesection 10C of the loop being secured to end section 13 of the frame,pipe section 10D of the loop being secured to end section 14D of theframe.

Coupling flange 15 of the loop is joined to the end flange of anupstream pipe 17 of a line conveying the fluid to be metered, whilecoupling flange 16 of the loop is joined to the end flange of thedownstream pipe 18 of the fluid line. Since the meter is bi-directionaland will operate with fluid flow in either direction, in practice pipe17 may serve as the downstream pipe, in which event pipe 18 is theupstream pipe.

Mounted at about the vertex of loop 10 by means of a releasable clamp 19or similar means is an electromagnetic ballistic vibrator 20 energizedby an oscillator 21 external to the assembly to vibrate at a frequencywhich corresponds to the natural resonance frequency of the loop. Thiscauses the loop to oscillate at its resonance frequency, the loopswinging back and forth in a direction normal to the static verticalplane thereof.

As shown separately in FIG. 3, a preferred form of ballistic vibratorconsists of a three-legged ferromagnetic stator 22 whose center leg hasa coil 23 wound thereon, so that when an alternating voltage fromoscillator 21 is applied thereto, during the positive half of eachcycle, the center leg is polarized, say, North, while the end legs areboth polarized South. This polarization is reversed during the negativehalf of the cycle.

Cooperating with the stator which is secured to the loop vertex is amovable mass constituted by a U-shaped permanent magnet 24, one of whoselegs is polarized North, and the other South. The permanent magnet isfree to move axially within the space defined by the end legs of thestator, one leg of the magnet lying between the center leg of the statorand the left end leg thereof, the other leg of the magnet lying betweenthe center leg of the stator and the right end leg thereof.

With the polarization shown in FIG. 3 which exists during the positivehalf of each cycle, the North leg of magnet 24 is attracted toward theSouth left end leg of stator 22 while being repelled by the North centerleg of the stator. At the same time, the South leg of the magnet isattracted to the North center leg of the stator while being repelled bythe South right leg of the stator; hence, the magnet is then drivenaxially in the left direction. During the negative half of the cycle,the polarization of the stator legs is reversed, and the magnet is thendriven in the right direction. The axis of the magnet is at right anglesto the static vertical plane of the loop, and the loop is thereforecaused to swing on either side of this plane when the ballistic vibratoris energized.

Mounted near the lower ends of legs 10A and 10B of the loop in balancedrelation is a pair of strain gauge transducers 25A and 25B. A straingauge is a device which senses mechanical deformation and is normallyattached to the structural element being deformed. Some types of straingauges exploit a change of electrical resistance of a wire or a siliconsemiconductor under tension, the gauge converting a small mechanicalmotion into an electrical signal by virtue of the fact that when thewire or semiconductor is stretched, its resistance is changed.Preferably, use is made of a piezoelectric strain gauge transducer whichgenerates a signal as a function of the strain to which the gauge issubjected.

The signal from strain gauge transducers 25A is applied to apre-amplifier 26 and that from strain gauge transducer 25B to apre-amplifier 27. The output of pre-amplifier 26 is connected to thenegative input of a differential amplifier 28 through a fixed resistor29 in series with a variable gain-control resistor 30. The output ofpre-amplifier 27 is connected to the positive input of differentialamplifier 28 through a fixed resistor 31. The output of differentialamplifier 28 which represents the difference between the amplitudes ofthe strain gauge signals is applied to a micro-controller 32 whichincludes a microprocessor whose function will be later explained.

The output of pre-amplifier 26 is also applied through a fixed resistor33 to the input of a summing amplifier 34 to which is also appliedthrough a fixed resistor 35 the output of pre-amplifier 27. Hence, theoutput of summing amplifier 34 is the sum of the strain gauge signals,and this is applied to another input of microcontroller 32.

Microcontroller 32, on the basis of the sum and difference signal dataentered therein, calculates the mass flow rate of fluid flowing throughthe flow loop to provide a digital value representing the mass flowrate. This is displayed on visual indicator 36.

Frame 11 is heavy in relation to loop 10, so that very little of theloop vibration is transmitted to frame 11. To minimize frame vibration,the frame could be attached to an external supporting member. However,in large meters where frame vibration may be troublesome, a ballisticvibrator 37 is attached to the midsection 12 of the frame, and thisvibrator is energized by a-c power supplied thereto by microcontroller32 so that vibrator 37 is energized at the same frequency as the loopresonance frequency, but out of phase therewith so as to dampen thevibration of the frame.

Operation

We shall first consider the behavior of the meter at zero flow whenballistic vibrator 20 is powered to cause loop 10 to vibrate at itsresonance frequency. As a consequence, loop 10, which is shown in itsstatic vertical solid lines in FIGS. 4 and 5, then swings back andforth, so that the loop first occupies the position shown by dashed lineloop 10X on one side of the static plane, and by dashed line loop 10Y onthe other side. At zero flow, strain gauge transducers 25A and 27Bshould ideally yield the same output, and if the respective gains ofpre-amplifiers 26 and 27 are the same, then the output of differentialamplifier 28 should be zero for incoming sensor signals of equal value.

In the event the strain gauge transducers are slightly mismatched, orthe loop slightly unbalanced, one can adjust the position of ballisticvibrator 20 on the loop and/or make a fine adjustment in the output ofpre-amplifier 26 by varying resistor 30 so that the output ofdifferential amplifier 28 is zero at no flow.

It is important to recognize that acceleration acts equally on bothsensors 25A and 25B and therefore shows up as common mode to produce nooutput in differential amplifier 28.

The output of summing amplifier 34 is proportional to angle α (see FIG.6), which depends on the amplitude of the resonance frequency vibrationof the loop. Since this amplitude is relatively constant for a given setof density, temperature and pressure conditions, short term accelerationeffects can be filtered out by using a relatively long time constant forthis parameter in microcontroller 32.

When there is fluid flow in vibrating loop 10, the resultant Coriolisforces causes the loop to twist and torsionally oscillate about a torqueaxis T. The loop then assumes an angle θ with respect to the loop in thestatic plane. This is indicated in FIG. 5 by dashed line loops 10X' and10Y'. Angle θ of this torsional oscillation is proportional to mass flowas long as the frequency of oscillator 21 and the amplitude of loopvibration remain constant.

This twisting motion changes the relative amplitudes of the signals fromstress gauge sensors 25A and 25B, for it increases stress on one leg ofthe loop while decreasing stress on the other leg. The resultant outputof differential amplifier 28 represents the difference between thestress sensor signals and is proportional to mass flow as long as thefrequency of oscillator 21 and the amplitude of loop vibration remainconstant.

The microprocessor in microcontroller 32 therefore acts to accept themass flow rate signal from differential amplifier 28. It corrects thisflow rate signal for the vibration angle represented by the output ofsumming amplifier 34 and the frequency of oscillator 21. As aconsequence, the mass flow rate value yielded by microcontroller 32 isan accurate indication thereof.

While there has been shown and described a preferred embodiment of aCORIOLIS-TYPE MASS FLOWMETER in accordance with the invention, it willbe appreciated that many changes and modifications may be made thereinwithout, however, departing from the essential spirit thereof. Thus,instead of arranging loop 10, as shown in FIG. 1, so that the staticplane of the loop lies parallel to the longitudinal axis of the frame11, it may be arranged to be at right angles to this axis.

I claim:
 1. A mass flowmeter of the Coriolis type comprising:A a flowloop formed by a pipe shaped to define at the lower end of the loop aninlet and an outlet extending in opposing directions, the midpoint ofthe loop at its lower end being intermediate the inlet and the outletand lying on the torsion axis of the loop; B a stationary framesupporting the loop whereby it is free to vibrate at its naturalresonance frequency and to torsionally oscillate about the torsion axis,said frame having a pair of end sections secured to the inlet and theoutlet, respectively, and a midsection secured to said midpoint inalignment with said torsion axis; C means coupled to the upper end ofthe loop to cause it to vibrate at its natural resonance frequency; Dmeans to interpose said loop in a line conducting the fluid to bemetered whereby the fluid enters the inlet and is discharged from theoutlet to cause the vibrating loop to undergo torsional oscillationabout said midpoint secured to said midsection as a function of massflow; and E means including sensors coupled to the loop to sense saidtorsional oscillation to provide a signal indicative of mass flow.
 2. Aflowmeter as set forth in claim 1, wherein said means to cause said loopto vibrate is constituted by an electromagnet.
 3. A flowmeter as setforth in claim 1, wherein said loop includes a first arcuate legadjacent said inlet and a second arcuate leg adjacent said outlet, andwherein a pair of sensors is provided, one mounted on the first leg andthe other on the second leg.
 4. A flowmeter as set forth in claim 3,wherein said sensors are strain gauges whereby in the absence of flowthe gauges yield equal signals and during flow they yield signals ofdifferent magnitude as a function of mass flow.
 5. A flowmeter as setforth in claim 4, further including means to apply the signals from thegauges to a differential amplifier whose output is substantiallyproportional to the mass flow rate, and means responsive to thedifferential amplifier output to provide a mass flow rate reading.
 6. Aflowmeter as set forth in claim 1, wherein said loop is a single turnloop.
 7. A flowmeter as set forth in claim 1, wherein said inlet andsaid outlet each terminate in a coupling flange.